The mechanisms by which organs control cell proliferation to reach an appropriate final size during development and regeneration are a central question of biology, and are also critical to an understanding of cancer. Despite intensive work on this question, we currently appreciate only a fraction of these mechanisms. Comprehensive identification of organ growth control pathways will be required as a precursor to 'systems'-level understanding, and will also open up new avenues for manipulation and clinical intervention. Drosophila has become a favorite model system for understanding the basic cellular functions and intercellular interactions by which organ dimensions are determined. The identification of Drosophila 'tumor suppressor genes (TSGs)', mutations in which cause cells and/or organs to overproliferate, has provided both mechanistic insight into known pathways as well as the identification of completely new organ size control pathways that appear conserved in vertebrates. Several extensive screens for fly TSGs have been carried out, but these have a significant blind spot: they rely on survival and appropriate differentiation of the cells in the adult, as well as adult survival of the tumor- containing animal. We have designed and executed a novel genetic screen ('MENE') that isolates a previously inaccessible set of potent TSGs, mutations in which cause lethal and often disorganized overgrowth of poorly differentiated imaginal disc cells ('neoplastic TSGs'). In this proposal, we will use new mutants from the MENE screen as an entry point to study two distinct cellular functions with unappreciated roles in restraining tissue growth. The first involves epigenetic control of growth by the Polycomb Group (PcG) family of chromatin regulators. The second involves a ubiquitin ligase that regulates both cell proliferation and epithelial polarity. We will determine the mechanism of growth restraint for these TSGs, and integrate their activities into known pathways controlling disc growth. Finally, we will extend the genome-wide screen for neoplastic TSGs, in order to identify additional pathways that restrain disc cell proliferation and elucidate the common mechanisms that underlie all neoplastic TSG activities. Together, these studies will advance our long-term goal of understanding the entire constellation of cellular processes by which organs control their growth and prevent tumor formation. PUBLIC HEALTH RELEVANCE: The mechanisms by which organs control cell proliferation to reach an appropriate final size are a central question of biology, and are also critical to an understanding of cancer. Drosophila provides a simple model system ideal for unbiased, genome-wide identification of genes that prevent cellular overproliferation. This proposal will elucidate how two new and unappreciated cellular mechanisms function and interact with other known mechanisms to ensure proper tissue growth in the fly, and identify further novel growth-control mechanisms that are likely to be conserved in humans.